feat: Audio Preview Player — browse and play WAV samples via Web Audio API

Issue: #9

Backend:
- GET /api/list-wavs?dir=<path> — lists .wav files in a directory
- GET /wavs/<file> — static file serving from ./output/wavs

Web UI:
- WAV Browser section with path input + Browse button
- AudioContext-based playback (zero deps) — progress bar, time display
- WAV file grid with Play buttons, active item highlighting

Tests:
- 3 new tests for handleListWavs (wav files, empty dir, nonexistent dir)
- Fix TestFindAkaiutil: add t.Chdir(tmp) so relative path lookups don't
  accidentally find the repo's third_party/akaiutil/akaiutil binary

serve_test.go

@@ -442,4 +442,68 @@ func TestHandleAPIExtract_ExtractISOError(t *testing.T) {
 	if !strings.Contains(msg, "FAIL bad.iso") {
 		t.Errorf("expected FAIL message, got: %s", msg)
 	}
+}
+
+func TestHandleListWavs_ReturnsWavFiles(t *testing.T) {
+	tmp := t.TempDir()
+	wavDir := filepath.Join(tmp, "wavs")
+	if err := os.MkdirAll(wavDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	for _, name := range []string{"kick.wav", "snare.wav", "hihat.wav"} {
+		fd, _ := os.Create(filepath.Join(wavDir, name))
+		_ = fd.Close()
+	}
+	fd, _ := os.Create(filepath.Join(wavDir, "readme.txt"))
+	_ = fd.Close()
+
+	req := httptest.NewRequest("GET", "/api/list-wavs?dir="+wavDir, nil)
+	rr := httptest.NewRecorder()
+	handleListWavs(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d", rr.Code)
+	}
+	var files []string
+	if err := json.Unmarshal(rr.Body.Bytes(), &files); err != nil {
+		t.Fatalf("invalid JSON: %v", err)
+	}
+	if len(files) != 3 {
+		t.Errorf("expected 3 wav files, got %d: %v", len(files), files)
+	}
+}
+
+func TestHandleListWavs_EmptyDir(t *testing.T) {
+	tmp := t.TempDir()
+	req := httptest.NewRequest("GET", "/api/list-wavs?dir="+tmp, nil)
+	rr := httptest.NewRecorder()
+	handleListWavs(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d", rr.Code)
+	}
+	var files []string
+	if err := json.Unmarshal(rr.Body.Bytes(), &files); err != nil {
+		t.Fatalf("invalid JSON: %v", err)
+	}
+	if len(files) != 0 {
+		t.Errorf("expected 0 files, got %d", len(files))
+	}
+}
+
+func TestHandleListWavs_NonExistentDir(t *testing.T) {
+	req := httptest.NewRequest("GET", "/api/list-wavs?dir=/nonexistent/wavs", nil)
+	rr := httptest.NewRecorder()
+	handleListWavs(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d", rr.Code)
+	}
+	var resp map[string]any
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("invalid JSON: %v", err)
+	}
+	if _, ok := resp["error"]; !ok {
+		t.Error("expected error for nonexistent dir")
+	}
 }